I designed my garage around an 8' garage door. I found the height of the door and added enough for the header over the door. The header also allowed enough height to hold the door when open. Overall, its about 9'. I love the extra height and the 8' tall doors. I sided with T-111, and the only issue was not being able to find 9' tall plywood.
If you have the space, I'd go with a 9' foot door. A standard 8' is too narrow many times. I put in a double 16' door and a single 9' door. My garage is 28x42, but then I have 20 acres, so space wasn't an issue.
Random thoughts:
If you can, put in some skylights to allow light into the shed from above.
I had the gambrel trusses built locally also. Worked great for putting them up since they became the floor for the 2nd story and the roof in an afternoon.
You will probably also want to hang tools from the rafters to get them out of the way, as well as store supplies along the outer edges. Allow for this when sizing.
Good luck on the project.